In many types of electrical machine, electric currents in a stator create a travelling magnetic field that interacts with a set of electric currents, a set of permanent magnets or a set of ferromagnetic features on the moving part of the machine, i.e. a rotor in the case of a rotary machine, or a translator in the case of a linear machine. The usual method of creating the travelling field is to use three component windings housed together in a uniform array of slots in a laminated iron stator and fed with three alternating currents from a three-phase electrical power supply. Each component winding has a cyclic distribution of coils and the three component windings are placed with their magnetic axes spaced at intervals of ⅓ of a wavelength along the stator. The alternating currents fed to the windings have relative phase differences of 120 degrees. As a result, the combined magnetic flux approximates a sinewave of constant amplitude travelling at a speed equal to one wavelength of the cyclic distribution during one cycle of the alternating current. Other polyphase windings are possible but are seldom used because the prevalent form of power supply is the three-phase type.
The most common type of electrical drive, the three-phase induction motor of the type used in many industrial applications, employs this method to produce a rotating field. Referring to FIG. 1(a), a rotor 12 carries a set of conducting bars (not shown) in which electric currents are induced and which interact with the rotating field to produce torque on an output shaft.
In domestic and light industrial applications, where a three-phase electrical supply is not available, an alternative arrangement is used whereby two component windings are supplied with alternating currents differing in phase. The first current is provided directly by an available single-phase supply and the second is obtained from the same supply usually via a capacitor that introduces a phase shift. The phase shift is of the correct degree only under one load condition so under most conditions the operation of such motors is not ideal. Such machines are referred to as single-phase motors because they operate from a single-phase supply but the windings are more accurately described as two-phase windings. If a balanced two-phase supply providing two currents displaced in phase by 90 degrees were available, electrical machines of this type with component windings displaced by ¼ of the wavelength would create a rotating field of constant amplitude and could be just as effective as standard three-phase machines.
In recent years, linear electric motors have attracted interest for several applications including guided ground transport and for electromagnetic launch systems on aircraft carriers. Also linear generators have been used in certain wave-power devices relying on reciprocating movement. A linear electrical machine can be considered as a standard rotary machine that has been cut and un-rolled, as shown in FIG. 1(a) . . . (c). When this is done, the magnetic force of attraction indicated by the force lines 16 between stator 10 and translator 14 is no longer balanced by equal and opposite forces as in the case of the stator 10 and rotor 12.
A common approach to overcome this problem is to use two stators 10′, 10″ placed on opposite sides of a single translator 14 as shown in FIG. 2. In most cases magnetic flux 18 passes from the first stator 10′, through the translator 14, through the second stator 10″, and through the translator for a second time to complete its circuit.
The two-stator linear machine could be rolled up around the original axis (shown as A in FIG. 1(b)) to form a rotary machine with two co-axial stators enclosing a hollow cylindrical rotor and with magnetic flux passing radially from the inner stator, through the inner gap, the cylindrical rotor and the outer gap to the outer stator. Machines of this configuration have applications in servo control systems where a drive of very low inertia is required.
Alternatively, the linear machine may be rolled up around an axis orthogonal to the original axis (shown as B in FIG. 2) to create a rotary machine where the flux passes axially across two planar airgaps rather than radially through cylindrical airgaps. Axial-flux rotating machines of this type have been used particularly as permanent-magnet generators for renewable energy applications, notably small wind turbines. In such cases each of the two stators normally carries a three-phase winding comprising three component windings as described above.
It is an object of the present invention to provide an improved electrical machine.